This invention relates to a method for welding a packing band made of a thermoplastic resin and the resulting welded packing band. More particularly, the invention relates to a method for connecting the ends of a flat band of thermoplastic resin, preferably of polyolefin or polyethylene terephthalate, in which method the band is wound around an article to be packed and both ends of the band are made molten and firmly pressed against each other to form a weld therebetween, and also to the structure of the thus connected band.
A band of thermoplastic resin is commonly used for packing various articles. The band of this kind has a flat cross section and a high tensile strength. While the band of thermoplastic resin has been used for packing corrugated cardboards for a long time, it recently has been also used for packing articles of heavy weight.
When the band of thermoplastic resin is used for packing, the band is at first wound around an article to be packed, then tightened so as not to leave any loose part or slack in the wound band, and in this state both ends of band are connected to each other. The both ends must be firmly connected so that the band will neither slip nor separate. Especially, when the band is used for packing an article of heavy weight, it is very important that the band ends are firmly attached to each other. One reason for this is that when a number of articles of heavy weight are stacked one on the other, the band packing the lower articles can be broken due to the large amount of heavy weight exerted thereon, resulting in collapse of the articles.
In the prior an clasps have been used for connecting the band. However, when it is considered that the band ends made of thermoplastic resin can be firmly connected together by heating to form a weld therebetween, connecting the ends of the band by heating and welding is practical for thermoplastic bands. When the band of a thermoplastic resin (hereinafter referred to as a "band") has been connected at both ends thereof, various methods for welding the band have been employed. The method for welding the band is classified depending on particular process used for melting the thermoplastic resin constituting the band. The processes for melting the resin can include a process using a heating plate, a process using frictional heat, and a process using ultrasonic waves.
When an article of heavy weight is packed with the band, no matter whatever method is used for welding the band, it is difficult to obtain a welded part of the band having a strength of more than 60% of the strength of the band itself prior to welding. More particularly, when a band having a tensile strength of more than 300kg/cm.sup.2 is used for packing an article and such a band is welded for packing, the resulting welded part has a tensile strength only within the range 30-60% of that of the band itself prior to welding. Thus, the resulting welded band is not suitable for packing articles of heavy weight, even though the band itself has a great tensile strength.
When the band is connected by welding, the welding has previously been conducted in manner such that the welded portion forms a single continuous area on the band which extends in the lengthwise direction of the band. When it is desired to increase the strength of the welded part, it has been common practice to increase the area of the welded part of the band. In this case, when viewed from the widthwise direction of the band (the transverse direction relative to the length of the band), it has also been practiced to weld the band over its entire width, namely, completely from one side to the other. Thus, in order to increase the strength of welded part of the band, the welded part has only been extended in the lengthwise direction of the band in a manner such that the welded part forms a single continuous area.